Because of their high resistance to both corrosion and rolling contact fatigue, silicon nitride bearing balls (or "roller elements") have replaced steel roller elements in many high performance bearing applications. First generation silicon nitride-based bearings represented only a partial replacement of silicon nitride for steel. That is, these bearings typically comprised a plurality of silicon nitride roller elements housed within an inner steel ring and an outer steel ring. However, over time, the same desire to improve the corrosion and rolling contact fatigue of the inner and outer rings led to the development of the "all ceramic" bearing, in which each of the roller elements, the inner ring and the outer ring comprised ceramics.
Typically, in all-ceramic bearings, the same silicon nitride material has been employed for each component. See, for example, U.S. Pat. No. 5,575,571 and U.S. Pat. No. 4,634,300 (col.3, line 65). One reason for using identical materials in each component is provided in U.S. Pat. No. 4,968,158, which teaches that the bearing and race materials should have the same coefficient of thermal expansion.
The introduction of such "all-ceramic" bearings has not been without problems. U.S. Pat. No. 4,770,549 (Rokkaku) noted that, in some applications, all ceramic bearings in which the roller elements and rings were made of the same material were prone to exfoliation (or "spalling"). Rokkaku teaches a solution of an all-ceramic bearing in which the fracture toughness (K.sub.1C) of the roller elements exceeds that of the inner and outer rings by about 0.5-2.0 MPa m.sup.1/2. According to Rokkaku, since the roller elements are more prone to spalling, their rolling contact fatigue ("RCF") lives are improved by having a higher fracture toughness.
In simulation RCF testing of similar materials, although it has been found that although the specific ceramic-ceramic combinations of Rokkaku do in fact have a higher spalling resistance than those combinations of identical materials, these combinations (jn which the ball is made of a tougher material) nonetheless display high wear.
The Rokkaku patent discusses above generally used silicon nitride materials having a relatively low fracture toughness (i.e., between 5 and 6 MPa m.sup.1/2) U.S. Pat. No. 5,575,571 ("Takebayashi") discloses a ceramic-ceramic bearing in which both the roller elements and the race are made of a silicon nitride material having a hardness of about 15 GPa and a fracture toughness of above 6.2 MPa m.sup.1/2.
Again however, in simulation RCF testing of similar materials, it has been found that although these higher toughness materials do in fact have a higher spalling resistance than those combinations of identical materials having lower toughness, these combinations (of identical high toughness materials) also display high wear.
Thus, in addition to spalling resistance, wear resistance is also a critical factor in the performance of an all-ceramic bearing. In general, it is desirable for a bearing to have low wear because excessive wear causes both a loss of preload and an increase in the ball-race clearance which leads to undesirable sliding. In addition, increasingly worn regions are more prone to spalling as well. In general, the wear of a ceramic-ceramic bearing is a complex function of many variables, including toughness and hardness. All else being equal, the wear resistance of the bearing increases with increased hardness and increased toughness of the contact components. Also generally, an increase in the hardness of a ceramic material generally corresponds to a decrease in its toughness.
Therefore, it is the object of the present invention to provide an all-ceramic bearing having high resistance to both spalling and wear.